I am using Irish Central Bank Rates to calculate value in Euro for foreign currencies. For example, if you buy something in UK and you have invoice/receipt in Sterling (£), you can find the date of purchase in table below and use the rate to calculate Euro value for accounts purposes.

For example, if you pay by credit card the better way is to use the rate on your Credit card statement. If you pay by cash and you exchanged Eur to Sterling, you can use rate the Exchange office charged you. Otherwise, if you cannot identify the exchange rate, you can use table below.

FX rate = foreign exchange rate

Source:
Foreign exchange rates from Central Bank of Ireland from 2008 to present (MS Excel document)

Foreign exchange rates 2017

January February March April May
June July August September October
November December
Date Pound sterling US dollar Czech koruna Polish zloty
2 Jan 17 0.85140 1.0465 27.021 4.4123
3 Jan 17 0.84565 1.0385 27.020 4.3934
4 Jan 17 0.84945 1.0437 27.021 4.3778
5 Jan 17 0.85440 1.0501 27.021 4.3680
6 Jan 17 0.85648 1.0589 27.021 4.3552
9 Jan 17 0.86660 1.0516 27.021 4.3777
10 Jan 17 0.86940 1.0567 27.021 4.3721
11 Jan 17 0.86725 1.0503 27.021 4.3703
12 Jan 17 0.86843 1.0679 27.021 4.3765
13 Jan 17 0.87548 1.0661 27.021 4.3739
16 Jan 17 0.87808 1.0594 27.021 4.3729
17 Jan 17 0.86790 1.0684 27.021 4.3693
18 Jan 17 0.86833 1.0664 27.021 4.3673
19 Jan 17 0.86555 1.0668 27.021 4.3728
20 Jan 17 0.86595 1.0632 27.021 4.3699
23 Jan 17 0.86148 1.0715 27.027 4.3719
24 Jan 17 0.86350 1.0748 27.021 4.3725
25 Jan 17 0.85323 1.0743 27.022 4.3641
26 Jan 17 0.85143 1.0700 27.021 4.3435
27 Jan 17 0.85170 1.0681 27.021 4.3400
30 Jan 17 0.84935 1.0630 27.022 4.3310
31 Jan 17 0.86105 1.0755 27.021 4.3239
Date Pound sterling US dollar Czech koruna Polish zloty
1 Feb 17 0.85418 1.0790 27.021 4.3186
2 Feb 17 0.86068 1.0808 27.022 4.3071
3 Feb 17 0.86013 1.0741 27.021 4.2972
6 Feb 17 0.85950 1.0712 27.021 4.2767
7 Feb 17 0.86330 1.0675 27.021 4.3035
8 Feb 17 0.85315 1.0665 27.021 4.3118
9 Feb 17 0.85090 1.0692 27.021 4.3096
10 Feb 17 0.85290 1.0629 27.021 4.2953
13 Feb 17 0.84890 1.0629 27.021 4.3137
14 Feb 17 0.85098 1.0623 27.021 4.3012
15 Feb 17 0.84998 1.0555 27.021 4.3015
16 Feb 17 0.85110 1.0652 27.021 4.3176
17 Feb 17 0.85720 1.0650 27.021 4.3305
20 Feb 17 0.85145 1.0616 27.021 4.3246
21 Feb 17 0.84860 1.0537 27.021 4.3064
22 Feb 17 0.84450 1.0513 27.021 4.2987
23 Feb 17 0.84628 1.0573 27.021 4.3080
24 Feb 17 0.84503 1.0609 27.021 4.3107
27 Feb 17 0.85280 1.0587 27.021 4.3148
28 Feb 17 0.85305 1.0597 27.021 4.3148
Date Pound sterling US dollar Czech koruna Polish zloty
1 Mar 17 0.85550 1.0533 27.021 4.2958
2 Mar 17 0.85560 1.0514 27.021 4.2941
3 Mar 17 0.86355 1.0565 27.021 4.3007
6 Mar 17 0.86300 1.0592 27.021 4.3091
7 Mar 17 0.86710 1.0576 27.021 4.3075
8 Mar 17 0.86753 1.0556 27.021 4.3130
9 Mar 17 0.86653 1.0551 27.021 4.3171
10 Mar 17 0.87250 1.0606 27.021 4.3259
13 Mar 17 0.87258 1.0663 27.021 4.3376
14 Mar 17 0.87563 1.0631 27.021 4.3221
15 Mar 17 0.87073 1.0622 27.021 4.3206
16 Mar 17 0.86810 1.0726 27.021 4.2974
17 Mar 17 0.86868 1.0737 27.021 4.2982
20 Mar 17 0.86793 1.0752 27.021 4.2820
21 Mar 17 0.86753 1.0802 27.021 4.2628
22 Mar 17 0.86790 1.0807 27.021 4.2816
23 Mar 17 0.86273 1.0786 27.021 4.2685
24 Mar 17 0.86600 1.0805 27.021 4.2695
27 Mar 17 0.86403 1.0889 27.021 4.2641
28 Mar 17 0.86455 1.0859 27.021 4.2470
29 Mar 17 0.86385 1.0748 27.021 4.2389
30 Mar 17 0.86180 1.0737 27.022 4.2233
31 Mar 17 0.85553 1.0691 27.030 4.2265

Source:
Foreign exchange rates from Central Bank of Ireland from 2008 to present (MS Excel document)